In the case where molten pig iron is manufactured by using a blast furnace, iron ore and coke are charged into the blast furnace in order to pack each of them in the form of a layer; the iron ore and coke are heated with high-temperature hot air which is blown into the blast furnace through tuyeres; and the iron ore is reduced with CO gas which is generated from the coke. In order to stably operate the blast furnace, since it is necessary to achieve gas permeability and liquid permeability in the furnace, it is indispensable for the operation coke having various excellent properties such as strength, particle diameter, and strength after a reaction. In particular, coke strength such as drum strength is an important property.
The strength of coke is usually determined and controlled by performing a drum strength test prescribed in JIS K 2151 or the like. Coal particles melt and adhere to each other to form coke in a carbonization process. Therefore, the difference of thermoplasticity of coal strongly influences the strength of coke, and it is necessary to evaluate the thermoplasticity of coal from the viewpoint of controlling coke strength. “Thermoplasticity” refers to the quality of melting when coal is heated, and the thermoplasticity is usually evaluated in terms of fluidity, viscosity, adhesiveness, dilatability, permeability and/or the like of a thermoplastic material.
Typical example of a method for determining fluidity in a melting process among the indexes of the thermoplasticity of coal is a coal fluidity testing method using a Gieseler plastometer method prescribed in JIS M 8801. The Gieseler plastometer method involves: charging crushed coal particles having a particle diameter of 425 μm or less into a specified crucible; heating the sample at a specified heating rate; and determining the rotational speed of a stirring rod to which a specified torque is applied. The method indicates the thermoplasticity of the sample in terms of dial division per minute (ddpm). Known examples of other methods for evaluating thermoplasticity include a method for measuring torque by using a constant rotational speed method, a method for measuring viscosity by using a viscoelastometer, and a dilatometer method prescribed in JIS M 8801.
In contrast with a coal fluidity testing method, Patent Literature 1 proposes a method for evaluating thermoplasticity under a condition in which an environment to which thermoplastic coal is exposed in a coke oven is considered. That is, the method takes consideration of a condition of simulating an environment where thermoplastic coal is constrained and where the thermoplastic coal moves and permeates into the surrounding defective structure. Patent Literature 1 describes that a permeation distance determined by using this method is the index of the thermoplasticity of coal different from those determined by using conventional methods. Patent Literature 1 indicates that the permeation distance influences coke strength and that there is a decrease in coke strength as a result of large pore defects being formed in the coke in the case where coal having an excessively large permeation distance is mixed in a coal blend for coke making.
In addition, Patent Literature 2 discloses a method which involves: specifying the range of permeation distance that may cause a decrease in the coke strength; and controlling the permeation distance of an individual coal brand which is used as a coking raw material separately or in a blend with other coal brands so as to be out of the range. The disclosed specific examples of a method for controlling permeation distance mainly include two methods. One of the examples is a method which involves mixing coal brands having different permeation distances to control permeation distance. This method is based on the fact that the permeation distance of a coal blend is almost equal to the average permeation distance of individual constituent coal brands weighted by the blending ratios of the constituent coal brands. The other one is a method which involves making coal subject to an oxidizing treatment or a weathering treatment. This method is based on the fact that it is possible to decrease the permeation distance of a raw material coal by adjusting the degree of oxidation by controlling oxidizing conditions such as temperature, time, and oxygen content. In addition, Patent Literature 3 discloses that a method for manufacturing high-strength metallurgical coke involves: specifying the range of permeation distance that may cause a decrease in the coke strength as is the case with Patent Literature 2; and controlling the particle diameter of a coal brand within the range to be smaller than a specified particle diameter before blending the coal brand in a coaking coal blend, to reduce the negative effect of the coal brand having a permeation distance which may cause a decrease in coke strength. At the result, the method manufactures high-strength metallurgical coke from the coal blend.